Silicon nitride is a material which is basically very suitable for turbo superchargers, turbines of jet engines and linings of rocket jets and combustion chambers by virtue of its strength and its corrosion resistance.
For the above-mentioned uses, two factors play an important role in addition to the mechanical load bearing capacity and corrosion resistance at high temperatures, namely the resistance to thermal shock and the high temperature resistance. In the silicon nitride structures at present available, which contain about 10% by weight of oxidic sintering additives, the values of these important material data are still insufficient for high thermal stresses on account of the oxidic glass phases which form at the grain boundaries when sintering takes place.
According to U.S. Pat. No. 4,007,049, the thermal shock resistance is considerably improved by lowering the modulus of elasticity. This aim has in some cases been achieved by the preparation of composites. These diphasic composites consist of a ceramic material having a high modulus of elasticity, e.g. mullite or Al.sub.2 O.sub.3, and a ceramic material with low modulus of elasticity, such as boron nitride (U.S. Pat. No. 4,304,870).
The mechanical properties of these composites are intermediate between those of pure components, i.e. the strength at room temperature decreases with increasing BN content in the present example and the thermal shock resistance increases correspondingly. In contrast to phase pure ceramics, no significant decrease in strength with increasing temperature is observed.
Similar results were obtained by Ruh et al (J.Am.Ceram. Soc. 1981, 64, 415, Mater.Sci.Eng.71 (1985), 159-164) when they investigated hot pressed composites consisting of silicon nitride and boron nitride or silicon carbide and boron nitride. Both the modulus of elasticity and the strength at room temperature decrease with increasing BN content. The dielectric constant .epsilon. is decreased by the addition of boron nitride and the thermal shock resistance is improved, as was to be expected. In both cases, the components are left in the material as discrete phases after hot pressing at 1750.degree. C. since the diffusion constants of these covalent compounds are negligibly small even when high pressures and temperatures are employed, and there is no solubility. In spite of careful homogenisation of the starting materials, therefore, microscopic inhomogeneities, which are partly responsible for the low strength of the composites, occur due to the random distribution of the primary particles in the ceramic parts.
One possible method of improving the homogeneity of such ceramics is described in EP-A-389 084, in which a soluble polyhydrosilazane is polymerised in an autoclave together with a soluble organic boron compound in a solvent.
The polyborosilazane obtained has a higher molecular weight than the original polysilazane due to the copolymerisation of the boron compound with the polysilazane. The copolymer is subsequently pyrolysed to form a boron-containing ceramic powder. This process does not, of course, result in absolutely homogeneous distribution of boron in a silicon nitride ceramic since the polyhydrosilazane put into the process, which is a polymer, remains as a block during the reaction.
Another disadvantage is the narrow range of variation of the boron content in the ceramic since a starting polymer only has a small number of reactive centres compared with a monomer so that only a small amount of boron can be incorporated in the ceramic.
Compared with the desired ceramic which should consist only of silicon, boron and nitrogen, a ceramic produced according to EP-A-389 084 is composed at least of the elements silicon (about 40% by weight), boron (about 5% by weight), carbon (about 2% by weight), nitrogen (about 35% by weight) and oxygen (about 12% by weight). The relatively high proportion of oxygen is in the form of amorphous boron silicate glass. This glass has a deleterious effect on the high temperature strength and resistance to temperature changes of a ceramic body on account of its low softening point and relatively high coefficient of expansion.
A similar attempt in order to produce a homogeneous material consisting of boron, nitrogen and silicon is described in EP-A 424,082, in which a soluble polyhydridosilazane is reacted with a borane-lewis-base complex.
Because of the reasons already mentioned before, the polyborosilazane obtained by this method cannot be converted into a ceramic material with a homogeneous distribution of boron in a silicon nitride matrix.
Another disadvantage are the expensive starting materials required for this process.
It is an object of the present invention to provide novel organometallic precursor compounds which can be produced easily and in high yields with low cost starting materials and a process for the production of nitridic ceramics consisting only of Si, B and N from these precursor compounds. Further, the process should ensure completely homogeneous distribution of the participating elements and be free from the disadvantages described above.